The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities.

Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. As we have mentioned, food is the external energy source for humans. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources.

The distinct means of metabolizing specific subunits all have the same goal, the production of the primary cellular energy source: adenosine triphosphate.

Figure %: Chemical structure of ATP

As you can see in the figure above, ATP contains three phosphate groups. These groups are primarily responsible for ATP's role as an energy source. During metabolic reactions, these phosphate groups can be transferred from ATP to yield either adenosine diphosphate (ADP) or adenosine monophosphate (AMP).

ATP -> ADP + P + energy, or
ATP -> AMP + 2P + energy
The release of one or more phosphate groups is energetically favorable: the reaction produces energy. ATP can also undergo a reaction with water to yield ADP or AMP to release energy. The cell can use the energy produced from the breakdown of ATP for whatever purpose is necessary. Often, the energetically favorable breakdown of ATP is often coupled with another, energetically unfavorable reaction that is designed to drive the first reaction forward through the synthesis of additional ATP.

ATP synthesis is almost exactly opposite to the process by which ATP is broken down to produce energy: phosphate groups are brought in contact with either ADP or AMP. While this process is not as favorable, it is able to occur with the energy derived from metabolizing foods. In addition to ATP, there are a number of other reactive molecules that are involved in the production of cellular energy. These are called coenzymes and their role is to help transfer other chemical groups like hydrogens. Coenzymes work in conjunction with metabolic enzymes to drive metabolic reactions. Among these are nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A. We will discuss the specific roles of both these molecules more in following sections.